1. Field of the Invention
This invention relates to a switching device control circuit, specifically to a switching device control circuit having a thermal shutdown function.
2. Description of the Related Art
There has been known a semiconductor IC (Integrated Circuit) having the thermal shutdown function, which halts circuit operation when it reaches or exceeds a predetermined high temperature so that internal devices are protected from destruction or deterioration due to the high temperature (Refer to Japanese Patent Application Publication No. 2003-108241, for example.).
In this kind of IC, there are cases where the device to be protected and its control circuit are formed in separate ICs. That is, an IC 100 that includes a power device (a switching device driving a large operating current) and its control IC 200 are formed in separate IC chips, as in a structure shown in FIG. 14. The control IC 200 exchanges signals with the IC 100 that includes the power device so as to control a current flowing through the power device. In this case, a temperature detection unit 101 is incorporated in the IC 100 that includes the power device.
The control IC 200 is configured to halt the operation of the power device to protect it when the control IC 200 receives a temperature detection signal from the temperature detection unit 101 and judges that a temperature of the power device has reached or exceeded a predetermined high temperature.
While the temperature detection unit 101 is incorporated in the IC 100 that includes the power device in the structure shown in FIG. 14, a timer circuit 201 is incorporated in the control IC 200 in an example structure shown in FIG. 15. The timer circuit 201 measures duration of time during which the power device is placed in operation. The control IC 200 estimates the temperature of the power device in response to the time measured by the timer 201, and controls so as to halt the operation of the power device when it judges that the temperature of the power device has reached or exceeded the predetermined high temperature.
However, in the case where the thermal shutdown is executed at the receipt of the temperature detection signal from the temperature detection unit 101 as in the structure shown in FIG. 14, there is a possibility that the control IC malfunctions (mistakenly executes the thermal shutdown, for example) because of a noise in the temperature detection signal. Also, there is a problem that the number of terminals increases since the temperature detection unit 101 is incorporated in the IC 100 that includes the power device. That is, the IC 100 that includes the power device requires an output terminal for the temperature detection signal, while the control IC 200 requires an input terminal for the temperature detection signal.
In the case where the timer circuit 201 is incorporated in the control IC 200 as in the structure shown in FIG. 15, obtaining a good correlation between the temperature of the power device and the duration measured by the timer 201 is difficult. Therefore, it is difficult to estimate the temperature of the power device accurately to execute the thermal shutdown.